This invention relates to captive screws of the type generally used to attach an upper panel to a lower panel or frame wherein it is desired to keep the fastener in position without loose items of hardware. The captive screw is mounted on the upper panel, such that the screw stays attached to the panel even when the threads of the screw shaft are fully disengaged from a second panel to which the upper panel is attached.
The present invention is directed toward a new and improved captive screw which is fully retractable, has a low profile, and has a hand snap-in capability requiring no tools for installation. Full retractability, in the case of a captive screw, means that the upper panel to which the captive screw itself attaches, and the lower panel against which this panel is to be attached by the screw threads, can be essentially flush against each other prior to the tightening down of the screw. That is, the end of the screw, when the captive screw is disengaged from the lower panel, does not protrude, or protrudes a minimal amount, beyond the thickness of the upper panel. Of existing captive screws, none is fully retractable, has easy hand snap-in capability requiring no tools, and has the low profile design provided by a barbed-type snap-in mechanism.
Prior captive screws that have barbed-type snap-in capability are not fully retractable. Full retractability is an important feature for many captive screw applications. Full retractability allows for the threads of a captive screw attached to an upper panel to be fully disengaged from the threaded hole of a lower panel while the upper and lower panel are in full contact. That is, the screw threads of the captive screw extend beyond the lower surface of the upper panel when the captive screw is fully disengaged. Without full retractability, a problem known as "jack-out" may occur. Jack-out occurs where a captive screw that is not fully retractable is disengaged while one or more other fasteners attaching an upper panel to a lower panel are fully engaged.
For example, jack-out occurs where a panel, such as a printed circuit board assembly, has four captive screws that are not fully retractable located four corners of the board. When one of the captive screws is disengaged while the other three corners of the board are rigidly fixed down by the remaining captive screws, the corner of the printed circuit board will rise when the screw thread no longer retracts. Flexing of the board (or, if the board is very stiff, undue pressure or deflection of the captive screw itself) will occur. This can be particularly damaging in the case of a printed circuit board. Undue flexure of a printed circuit board assembly can break solder joints, crack and destroy electronic devices, and damage the printed circuit board itself.
Additionally, a fast and inexpensive method of installation of the captive screw is an important requirement. Prior captive screws with both a low and profile and full retractability do not have a means allowing for snap-in installation. This is important in several regards. Snap-in capability increases installation speed thus reducing costs associated with assembling articles using captives screws since no tools are involved. Additionally, installation of the captive screw may be accomplished at any stage of assembly of an article. For example, if the captive screw is placed on a door panel, the captive screw of the invention at hand accommodates installation after the door panel is hung. Installation at this stage of assembly is inefficient or difficult if a press or other tool is required.
Prior fully retractable captive screws with a low profile have material-specific designs, such as those made exclusively for installation on a sheet metal panel or exclusively for installation on a plastic panel, and/or require a special tool to install. A low profile is desirable in many situations where a captive screw is used. For example, if captive screws are to be installed on a printed circuit board with entirely low profile electronic devices, it is likely that the highest point on the board is the captive screw. A lower profile will allow for reduced clearance above the board, allowing for smaller mechanical assemblies.